Composite for anode active material and method of preparing the same

ABSTRACT

Provided are a composite for an anode active material and a method of preparing the same. More particularly, the present invention provides a composite for an anode active material including a (semi) metal oxide and an amorphous carbon layer on a surface of the (semi) metal oxide, wherein the amorphous carbon layer comprises a conductive agent, and a method of preparing the composite.

TECHNICAL FIELD

The present invention relates to a composite for an anode activematerial and a method of preparing the same.

BACKGROUND ART

In line with recent miniaturization and lightweight trends in electronicdevices, the miniaturization and weight reduction have also beenrequired for batteries acting as a power source. Lithium-based secondarybatteries have been commercialized as a battery that may beminiaturized, light weighted, and charged to high capacity, and thelithium-based secondary batteries have been used in portable electronicdevices, such as small video cameras, mobile phones, and notebooks, andcommunication devices.

Lithium secondary batteries, as an energy storage device having highenergy and power, may be advantageous in that capacities or operatingvoltages thereof are higher than those of other types of batteries.However, since the safety of the batteries may be problematic due to thehigh energy, there may be a risk of explosion or fire. In particular,since high energy and output characteristics are required for hybridvehicles that have recently been in the spotlight, it may be consideredthat the safety is more important.

In general, a lithium secondary battery is composed of a cathode, ananode, and an electrolyte, in which charge and discharge may bepossible, because lithium ions, which are discharged from a cathodeactive material by first charging, may act to transfer energy whilemoving between both electrodes, for example, the lithium ions areintercalated into an anode active material, i.e., carbon particles, anddeintercalated during discharging.

Meanwhile, since there is a continuous need for high-capacity batteriesdue to the development of portable electronic devices, research intohigh-capacity anode materials, such as tin (Sn) and silicon (Si), whichhave significantly higher capacity per unit mass than that of carbonthat is used as a typical anode material, have been actively conducted.In the case that Si or a Si alloy is used as an anode active material,volume expansion may increase and cycle characteristics may degrade. Inorder to address the above limitations, the Si or the Si alloy may bemixed with graphite to be used as the anode active material. However,since the graphite may be non-uniformly distributed during the mixing,the cycle characteristics and lifetime may degrade.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides a composite for an anode active materialhaving improved electrical conductivity and a method of preparing thesame.

Technical Solution

According to an aspect of the present invention, there is provided acomposite for an anode active material including: a (semi) metaloxide(metal(loid) oxide); and an amorphous carbon layer on a surface ofthe (semi) metal oxide, wherein the amorphous carbon layer comprises aconductive agent.

According to another aspect of the present invention, there is provideda method of preparing a composite for an anode active material includingpreparing a mixed solution by mixing a conductive agent, a polymer, anda (semi) metal oxide in a solvent; and performing a heat treatment afterspray drying the mixed solution.

Advantageous Effects

According to the present invention, since a surface of (semi) metaloxide is coated with an amorphous carbon layer including a conductiveagent, a composite for an anode active material having improvedelectrical conductivity may be provided. Also, a performance of asecondary battery may be improved by including the composite for ananode active material.

MODE FOR CARRYING OUT THE INVENTION

The present invention may provide a composite for an anode activematerial including a (semi) metal oxide (metal(loid) oxide) and anamorphous carbon layer on a surface of the (semi) metal oxide, whereinthe amorphous carbon layer comprises a conductive agent.

That is, in a composite for an anode active material according to anembodiment of the present invention, electrical conductivity may beimproved by coating a surface of (semi) metal oxide with an amorphouscarbon layer including a conductive agent by a heat treatment, and thus,a performance of a secondary battery may be improved.

First, in the composite for an anode active material according to theembodiment of the present invention, a type of (semi) metal of the(semi) metal oxide is not particularly limited so long as the (semi)metal does not adversely affect operating characteristics of a batterywhile exhibiting relatively higher hydrophilicity and polarity than theconductive agent and a material of the carbon layer according to theselection of the (semi) metal, and the (semi) metal may be silicon (Si)or tin (Sn). Specifically, the (semi) metal oxide may include one ormore selected from the group consisting of SiO_(x), AlO_(x), SnO_(x),SbO_(x), BiO_(x), AsO_(x), GeO_(x), PbO_(x), ZnO_(x), CdO_(x), InO_(x),TiO_(x), and GaO_(x) (where 0<x<2).

Also, the conductive agent may be selected from the group consisting ofa line-type conductive agent, a point-type conductive agent, and amixture thereof.

The line-type conductive agent may include one or more selected from thegroup consisting of carbon nanotubes, carbon nanofibers, and graphene.

Since the carbon nanotubes and the carbon nanofibers have excellentstrength and high resistance to fracture, the deformation of a currentcollector due to the repetition of charging and discharging or externalforce may be prevented and the oxidation of the surface of the currentcollector in unusual environments, such as high temperature andovercharging, of the battery may be prevented. Thus, the safety of thebattery may be significantly improved. Also, the graphene has a surfacearea of about 2,600 m²/g and an electron mobility ranging from 15,000cm²/Vs to 200,000 cm²/Vs, in which the graphene has relatively usefulproperties in comparison to other carbon materials. In particular, theelectron transfer rate of graphene is close to the speed of light, andthe reason for this is that electrons move like massless particles inthe graphene. The graphene may be prepared by a typical Scotch tapemethod, an epitaxy method using a silicon carbide insulator, a chemicalmethod using a reducing agent, and a method using a metal catalyst.

The point-type conductive agent may include one or more selected fromthe group consisting of carbon black, acetylene black, Ketjen black,channel black, furnace black, lamp black, and thermal black.

The line-type conductive agent and the point-type conductive agent maybe classified according to the shape of the conductive agent. However,in the present invention, the classification of the line-type conductiveagent and the point-type conductive agent may be in accordance with anaspect ratio (major axis length/minor axis length). For example, in thecase that the aspect ratio is 1.5 or more, the conductive agent may be aline-type conductive agent. In the case in which the aspect ratio isless than 1.5, the conductive agent may be a point-type conductiveagent.

The conductive agent may be selected from the group consisting ofcrystalline carbon, amorphous carbon, and a mixture thereof. In the casethat the conductive agent is amorphous carbon, the secondary battery mayhave high discharge capacity and excellent rate characteristics.However, irreversible capacity may be high, a charge and dischargeefficiency may decrease, and energy density may decrease due to lowvolume density and electrical conductivity. In contrast, in the case inwhich the conductive agent is crystalline carbon, the discharge capacityof the secondary battery may decrease. However, since the electricalconductivity and energy density may be excellent and the reversibilityof charge and discharge processes is relatively excellent in comparisonto the amorphous carbon, the crystalline carbon may be used by beingmixed with the amorphous carbon in order to maximize the advantages ofeach carbon.

An amount of the conductive agent may be in a range of 1 wt % to 20 wt %based on a total weight of the (semi) metal oxide. In the case that theamount of the conductive agent is less than 1 wt %, an effect due to theconductive agent may be insignificant. In the case in which the amountof the conductive agent is greater than 20 wt %, the capacity of thesecondary battery may decrease.

Also, in the composite for an anode active material according to theembodiment of the present invention, the amorphous carbon layer may bederived from an organic polymer component. Specifically, the amorphouscarbon layer may be formed by heat treating the organic polymercomponent.

In this case, the organic polymer component is not particularly limitedso long as it is a resin acting as a matrix. Typical examples of apolymer resin usable as a binder material for a typical secondarybattery may be a polymer including one or more selected from the groupconsisting of carboxymethyl cellulose (CMC), sucrose, polyacrylonitrile(PAN), polyvinylidene fluoride (PVDF), and polyvinyl alcohol (PVA).

That is, the composite for an anode active material of the presentinvention may be formed in a structure, in which the amorphous carbonlayer forms a matrix and the conductive agent is included as a filler inthe matrix.

The conductive agent included in the amorphous carbon layer is apoint-type or line-type conductive agent, wherein the conductive agentmay be disposed on the surface of the (semi) metal oxide. The conductiveagent may have a form, in which the conductive agent is dispersed on thesurface of the (semi) metal oxide due to a binder for a secondarybattery which is carburized on the surface of the (semi) metal oxide. Ina case where the binder for a secondary battery is absent, theconductive agent may agglomerate or may not be included on the surfaceof the (semi) metal oxide.

The entire or partial surface of the (semi) metal oxide may be coatedwith the amorphous carbon layer. Specifically, a carbon content of theamorphous carbon layer may be 2 parts by weight to 30 parts by weightbased on 100 parts by weight of the (semi) metal oxide. In the case thatthe carbon content of the amorphous carbon layer is less than 2 parts byweight, surfaces of (semi) metal oxide particles may not be uniformlycoated with carbon. In the case in which the carbon content of theamorphous carbon layer is greater than 30 parts by weight, an excessiveamount of carbon may be coated to reduce the capacity of the secondarybattery. In this case, when the carbon content of the amorphous carbonlayer is greater than 15 parts by weight based on 100 parts by weight ofthe (semi) metal oxide, the entire surface of the (semi) metal oxide maybe coated with a carbon layer. When the carbon content of the amorphouscarbon layer is less than 15 parts by weight, the partial surface of the(semi) metal oxide may be intermittently coated.

A ratio of a diameter of the amorphous carbon layer to a particlediameter of the (semi) metal oxide, for example, may be 11.5:10. Athickness of the amorphous carbon layer may be in a range of 5 nm to 70nm.

Also, the present invention may provide a method of preparing acomposite for an anode active material including preparing a mixedsolution by mixing a conductive agent, a polymer, and a (semi) metaloxide in a solvent; and performing a heat treatment after spray dryingthe mixed solution.

The polymer may be a binder material, and specifically, may include oneor more selected from the group consisting of CMC, sucrose, PAN, PVDF,and PVA. The carbon may be derived from the polymer.

Furthermore, an amount of the conductive agent may be in a range of 1 wt% to 20 wt % based on a total weight of the (semi) metal oxide. In thecase that the amount of the conductive agent is less than 1 wt %, aneffect due to the conductive agent may be insignificant. In the case inwhich the amount of the conductive agent is greater than 20 wt %, thecapacity of the secondary battery may decrease.

The solvent may include an organic solvent, such as N-methylpyrrolidone(NMP), dimethylformamide (DMF), acetone, and dimethylacetamide, and/orwater. The above solvents may be used alone or in a mixture of two ormore thereof.

Also, a viscosity of the solution including the conductive agent,polymer, and (semi) metal oxide may be 1,000 cps or less. In the casethat the viscosity is greater than 1,000 cps, a spray drying process maynot be performed due to high viscosity.

The spray drying may produce granular powder by substantially removingthe moisture of the solvent, and an amorphous carbon layer derived fromthe conductive agent and polymer may be uniformly distributed on thesurface of the (semi) metal oxide by the spray drying. The spray dryingmay be performed using a spray dryer which includes a drying chamber inwhich the spray drying is performed, a hot air inlet pipe that isconnected to the drying chamber and supplies hot air to the dryingchamber to remove a dispersant, an air outlet that is connected to thedrying chamber to exhaust air cooled during the spray drying, a rawmaterial feed pipe that penetrates a wall constituting the dryingchamber and supplies a raw material to the drying chamber to performspraying, and powder recovery pipes that are connected to the dryingchamber to recover powders formed in the drying chamber by the spraydrying. However, the present invention is not limited thereto.

Specifically, the spraying may be performed by injecting the mixedsolution at a rate of about 15 cc/min to about 25 cc/min, an inlettemperature in the drying chamber of the spray dryer of about 200° C. toabout 250° C., and an outlet temperature of about 60° C. to about 80° C.

Also, the heat treatment may be performed at a temperature ranging from600° C. to 1200° C., and an amorphous carbon layer, as a matrix, may beformed as the polymer, i.e., an organic binder, coated on the (semi)metal oxide is carburized by the heat treatment. Simultaneously, theconductive agent, as a filler in the matrix, may also be adhered to thesurface of the (semi) metal oxide. In the case that the heat treatmenttemperature is less than 600° C., the surface of the (semi) metal oxidemay be not coated with carbon because the polymer is not carburized. Inthe case in which the heat treatment temperature is greater than 1200°C., crystals of the (semi) metal oxide may grow due to the hightemperature.

Furthermore, the present invention may provide a secondary batteryincluding a cathode including a cathode active material; a separator; ananode including the composite for an anode active material of thepresent invention that is coated with an amorphous carbon layerincluding a conductive agent; and an electrolyte.

Since the secondary battery according to an embodiment of the presentinvention may include the composite for an anode active material of thepresent invention as an anode active material, the performance of thesecondary battery may be improved.

In this case, the anode, for example, may be prepared by coating ananode current collector with a mixture of an anode active material, aconductive agent, and a binder, and then drying the coated anode currentcollector. If necessary, the conductive agent may be further added. Inthis case, a total amount of the conductive agent used in thepreparation of the anode may be controlled to be 20 wt % or less. Thecathode may also be prepared by coating a cathode current collector witha cathode active material and drying the coated cathode currentcollector.

The separator is disposed between the cathode and the anode, and a thininsulating film having high ion permeability and mechanical strength maybe used as the separator. Since the current collectors, electrode activematerials, conductive agent, binder, filler, separator, electrolyte, andlithium salt are known in the art, the detailed descriptions thereof areomitted in the present specification.

The separator is disposed between the cathode and the anode to form abattery structure, the battery structure is wound or folded to put in acylindrical battery case or prismatic battery case, and then a secondarybattery is completed when the electrolyte is injected thereinto. Also,the battery structure is stacked in a bi-cell structure, impregnatedwith the electrolyte, and a secondary battery is then completed when theproduct thus obtained is put in a pouch and sealed.

Hereinafter, the present invention will be described in detail,according to specific examples. The invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein.

Example 1

0.1 g of carbon nanotubes as a line-type conductive agent and 4.5 g ofcarboxymethyl cellulose as a polymer were used, and 10 g of SiO was usedas a (semi) metal oxide. The carbon nanotubes, carboxymethyl cellulose,and SiO were mixed in water to prepare a mixed solution. Subsequently,the mixed solution was spray dried at a rate of about 15 cc/min to about25 cc/min, an inlet temperature in a drying chamber of a spray dryer ofabout 200° C. to about 250° C., and an outlet temperature of about 60°C. to about 80° C., and then heat treated at 700° C. to prepare acomposite for an anode active material, in which a surface of SiO wascoated with an amorphous carbon layer that was derived from carbonnanotubes and carboxymethyl cellulose.

Example 2

A composite for an anode active material, in which a surface of SiO wascoated with an amorphous carbon layer that was derived from carbon blackand sucrose, was prepared in the same manner as in Example 1 except that0.1 g of carbon black as a point-type conductive agent and 6 g ofsucrose as a polymer were used.

Example 3

A composite for an anode active material, in which a surface of SiO wascoated with an amorphous carbon layer that was derived from carbonnanotubes and PAN, was prepared in the same manner as in Example 1except that 0.1 g of carbon nanotubes as a line-type conductive agentand 14 g of PAN as a polymer were used.

Example 4

A composite for an anode active material, in which a surface of SiO wascoated with carbon that was derived from carbon black and PVA, wasprepared in the same manner as in Example 1 except that 0.1 g of carbonblack as a point-type conductive agent and 16 g of PVA as a polymer wereused.

Example 5 Preparation of Anode Slurry

The composite prepared in Example 1 was mixed with natural graphite,carbon black, and polytetrafluoroethylene at a weight ratio of 30:65:2:3to prepare an anode slurry.

Comparative Example 1 Preparation of Anode Slurry

An anode slurry was prepared in the same manner as in Example 5 exceptthat commercial silicon monoxide was used instead of using thecomposites prepared in Examples 1 to 4.

Example 6 Preparation of Secondary Battery

One surface of a copper current collector was coated with the anodeslurry prepared in Example 5 to a thickness of 65 μm, dried and rolled.Then, an anode was prepared by punching into a predetermined size.

LiPF₆ was added to a non-aqueous electrolyte solvent prepared by mixingethylene carbonate and diethyl carbonate at a volume ratio of 30:70 toprepare a 1 M LiPF₆ non-aqueous electrolyte solution.

A lithium foil was used as a counter electrode, a polyolefin separatorwas disposed between both electrodes, and a coin-type secondary batterywas then prepared by injecting the electrolyte solution.

Comparative Example 2 Preparation of Secondary Battery

A secondary battery was prepared in the same manner as in Example 6except that the anode slurry prepared in Comparative Example 1 was used.

Experimental Example 1 Lifetime Characteristics and CapacityCharacteristics Analysis

In order to investigate capacity characteristics and lifetimecharacteristics of the secondary batteries prepared in Example 6 andComparative Example 2 according to charge and discharge cycles, thesecondary batteries prepared in Example 6 and Comparative Example 2 werecharged at 0.1 C to a voltage of 5 mV and charged to a current of 0.005C at 5 mV under constant current/constant voltage (CC/CV) conditions at23° C., and then discharged at 0.1 C to a voltage of 1.5 V under aconstant current (CC) condition to measure capacities.

Thereafter, the secondary batteries prepared in Example 6 andComparative Example 2 were charged at 0.5 C to a voltage of 5 mV andcharged to a current of 0.005 C at 5 mV under constant current/constantvoltage (CC/CV) conditions, and then discharged at 0.5 C to a voltage of1.0 V under a constant current (CC) condition. This charge and dischargecycle was repeated 1 to 50 times. The results thereof are presented inTable 1 below.

TABLE 1 Examples Lifetime characteristics (%) Example 6 91.6 ComparativeExample 2 81.3

-   -   Lifetime characteristics: (discharge capacity in a 49th        cycle/first cycle discharge capacity)×100

As illustrated in Table 1, it may be understood that the lifetimecharacteristics of the secondary battery prepared in Example 6 using thecomposite, in which the surface of SiO was coated with the amorphouscarbon layer that was derived from carbon nanotubes and carboxymethylcellulose, as an anode active material were improved to about 10% ormore in comparison to the secondary battery prepared in ComparativeExample 2 using SiO as an anode active material.

It may be estimated that since the electrical conductivity was improvedby using the composite that included SiO having the surface coated withthe amorphous carbon layer including the conductive agent, the lifetimecharacteristics of the secondary battery was further improved.

INDUSTRIAL APPLICABILITY

According to the present invention, since a surface of (semi) metaloxide is coated with an amorphous carbon layer including a conductiveagent, a composite for an anode active material having improvedelectrical conductivity may be provided. Since the composite for ananode active material may improve lifetime characteristics of asecondary battery, the composite for an anode active material may besuitable for secondary batteries.

1. A composite for an anode active material comprising: a (semi) metaloxide; and an amorphous carbon layer on a surface of the (semi) metaloxide, wherein the amorphous carbon layer comprises a conductive agent2. The composite for an anode active material of claim 1, wherein the(semi) metal oxide comprises one or more selected from the groupconsisting of SiO_(x), AlO_(x), SnO_(x), SbO_(x), BiO_(x), AsO_(x),GeO_(x), PbO_(x), ZnO_(x), CdO_(x), InO_(x), TiO_(x), and GaO_(x), where0<x<2.
 3. The composite for an anode active material of claim 1, whereinthe conductive agent is selected from the group consisting of aline-type conductive agent, a point-type conductive agent, and a mixturethereof.
 4. The composite for an anode active material of claim 3,wherein the line-type conductive agent comprises one or more selectedfrom the group consisting of carbon nanotubes, carbon nanofibers, andgraphene.
 5. The composite for an anode active material of claim 3,wherein the point-type conductive agent comprises one or more selectedfrom the group consisting of carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, and thermal black. 6.The composite for an anode active material of claim 1, wherein theconductive agent is selected from the group consisting of crystallinecarbon, amorphous carbon, and a mixture thereof.
 7. The composite for ananode active material of claim 1, wherein an amount of the conductiveagent is in a range of 1 wt % to 20 wt % based on a total weight of the(semi) metal oxide.
 8. The composite for an anode active material ofclaim 1, wherein the composite for an anode active material has astructure, in which the amorphous carbon layer forms a matrix and theconductive agent is included as a filler in the matrix.
 9. The compositefor an anode active material of claim 1, wherein a carbon content of theamorphous carbon layer is 2 parts by weight to 30 parts by weight basedon 100 parts by weight of the (semi) metal oxide.
 10. The composite foran anode active material of claim 1, wherein the entire or partialsurface of the (semi) metal oxide is coated with the amorphous carbonlayer.
 11. The composite for an anode active material of claim 1,wherein a thickness of the amorphous carbon layer is in a range of 5 nmto 70 nm.
 12. A method of preparing a composite for an anode activematerial, the method comprising: preparing a mixed solution by mixing aconductive agent, a polymer, and a (semi) metal oxide in a solvent; andperforming a heat treatment after spray drying the mixed solution. 13.The method of claim 12, wherein the polymer comprises one or moreselected from the group consisting of carboxymethyl cellulose (CMC),sucrose, polyacrylonitrile (PAN), and polyvinyl alcohol (PVA).
 14. Themethod of claim 12, wherein an amount of the conductive agent is in arange of 1 wt % to 20 wt % based on a total weight of the (semi) metaloxide.
 15. The method of claim 12, wherein a viscosity of the solutioncomprising the conductive agent, the polymer, and the (semi) metal oxideis 1,000 cps or less.
 16. The method of claim 12, wherein the sprayingis performed by injecting the mixed solution at a rate of 15 cc/min to25 cc/min and an inlet temperature in a drying chamber of a spray dryerof 200° C. to 250° C.
 17. The method of claim 12, wherein the heattreatment is performed at a temperature ranging from 600° C. to 1200° C.18. A secondary battery comprising: a cathode including a cathode activematerial; a separator; an anode including the composite for an anodeactive material of claim 1; and an electrolyte.